1. Field of the Invention
This invention relates to a porous membrane for the separation of blood components and to a method for the manufacture thereof. More particularly, this invention relates to a porous membrane which, as means for permeation useful for the separation of blood components, for example, has high thermal stability enough for the structure and the permeating property thereof to defy adverse effects of the hysteresis of the heat used as in sterilization and to a method for the manufacture thereof.
2. Description of Prior Art
As membranes for permeation intended for the separation of blood into the components, i.e. cells and plasma, various porous membranes have been adopted for actual use. These porous membranes must be subjected to a treatment for sterilization when they are put to use in such medical application.
Heretofore, as porous membranes of this nature, regenerated cellulose membrane, cellulose acetate membrane, polyvinyl alcohol membrane, polysulfone membrane, and polymethyl methacrylate membrane have been used. The macromolecular membranes of such materials are not only deficient in mechanical strength, pore diameter, and ability of plasma treatment but also defective in resistance to heat and dimensional stability at elevated temperatures and, therefore, withstand sterilization by autoclaving with great difficulty. They have been sterilized, therefore, preponderantly by the use of ethylene oxide gas or gamma ray. The sterilization by other method than the autoclaving method, however, entails the problem of toxicity of residual ethylene oxide gas, the problem of heavy consumption of time of treatment, or the problem of deterioration of resin by gamma ray.
A porous polyethylene hollow fiber membrane i.e. a porous hollow fiber membrane made of high-density polyethylene having a density of at least 0.955 g/cm.sup.3, containing in the circumferential wall thereof numerous minute pores extending from the inner surface through the outer surface of the peripheral wall, oriented in the direction of length, and having a porosity of 30 to 90% has been proposed (Japanese Patent Laid-open SHO No. 58(1983)-75,555). This hollow fiber is produced by preparing a highly oriented crystalline unstretched hollow fiber, cold stretching this hollow fiber, and subsequently hot stretching the resulting stretched hollow fiber. Owing to this procedure, the minute pores are mechanically formed in the hollow fiber membrane. Moreover, these minute pores extend substantially straight from the inner surface through the outer surface of the peripheral wall and they possess a substantially uniform diameter. This hollow fiber membrane, therefore, is not allowed to acquire any large pore density per unit volume. As a natural consequence, the hollow fiber membrane inevitably acquires a small capacity for treatment per unit surface area. It is deficient in the performance proper for a membrane of permeation, as evinced by its susceptibility of breakage due to the stress of orientation. This hollow fiber membrane is a stretched membrane after all, although it is made of a polyolefin of high thermal stability. When it is exposed to the intense heat of the sterilization by autoclaving, therefore, it undergoes heavy shrinkage and suffers from heavy impairment of its properties as a membrane.
A permeable membrane produced by mixing a polymer such as crystalline polyolefin or polyamide which is stretchable and sparingly soluble in a solvent to be used with a compound partially compatible with the polymer and easily soluble in the solvent, molding the resultant mixture in the shape of a hollow fiber, treating the hollow fiber with the solvent, drying the wet hollow fiber, and stretching the dried hollow fiber monoaxially or biaxially by 50 to 1500% has been proposed (Japanese Patent Publication SHO No. 57(1982)-20,970). Since this membrane is stretched for the purpose of increasing the pore diameter, it possesses low mechanical strength and poor durability. Moreover since the pores on the two surfaces and inside the peripheral wall have a substantially uniform structure and the crystals of the polymer are coares, this membrane separates components of medium to high molecular weights with difficulty. It, therefore, exhibits poor performance for the sake of permeation. This membrane, on being sterilized by autoclaving, undergoes heavy thermal shrinkage.
An object of this invention, therefore, is to provide a novel porous membrane for separation of blood components and a method for the manufacture thereof. Another object of this invention is to provide a porous membrane excellent in thermal stability enough for the construction of membrane and the performance for permeation to defy the adverse effects of the hysteresis of the heat and, therefore, useful for the separation of blood components and a method for the manufacture thereof. Yet another object of this invention is to provide a method for the manufacture of a porous membrane, which method is capable of imparting to the porous membrane ample performance for permeation without subjecting the porous membrane to any additional stretching treatment.